The CPMs on event related things might be high enough to just rely on ads.
http://mixergy.com/evite-harry-lin/
"effective CPM, when I left the company, we got, we’d driven it up, the effective CPM was just under ten dollars. And on certain pages, certain classes of pages, we enjoyed a one hundred dollar CPM, on all those ads"
Depending on the price range I would. My fiancee and I already use a shared calendar but it is woefully equipped for the heavy use she and I put it through.
All I can tell you is that it's something I'd definitely use, and something many of my friends who I've told this to over the years would use. That's not saying much, sure, but if you could provide a valuable and useful service, the demand will certainly follow.